Campania, Italy is a region of Italy and it's capital is Naples. At the time of writing, Italy (and therefore Campania) was under Napoleon's control after the conquest of Italy in the Napoleonic Wars. The line "And Death sits brooding o'er Campania's plain" likely refers to all of the death and destruction that the region suffered before yielding to French control. Today, Campania is a premiere tourist destination and many visit to see its rich history: it is home to the archaeological sites of Pompeii, Herculaneum, Oplontis, Paestum, Aeclanum, Stabiae, and Velia and has a rich culture, especially in its food, music, and architecture.

 

"Venice the Adriatic weds in vain,

And Death sits brooding o'er Campania's plain;

O'er Baltic shores and through Hercynian groves,

Stirring the soul, the mighty impulse moves..." (Barbauld lines 269-272)